Passive low frequency RFID readers and tags use operating principles that are well-known to those of ordinary skill in the art, and that are described in extensive detail in several seminal inventions, including U.S. Pat. No. 1,744,036 to Brard, U.S. Pat. No. 3,299,424 to Vinding, U.S. Pat. No. 3,713,148 to Cardullo et al., and U.S. Pat. No. 5,053,774 to Schuermann et al., and in textbooks such as Finkenzeller, “RFID Handbook” (1st Edition, 1999). The disclosure of U.S. Pat. Nos. 1,744,036 3,299,424, 3,713,148 and 5,053,774 and the “RFID Handbook” is incorporated herein by reference in its entirety
In RFID systems of this type, the reader (also sometimes referred to as a scanner or interrogator) device generates a tag activation signal, and receives identification data signals from the electronic identification (EID) tag. Such a reader device can use separate transmit and receive antenna elements to perform these functions. Readers in which a single antenna performs both transmit and receive functions are typically economical and efficient, and are commonly used in low-frequency RFID reader designs.
An example of the circuitry of a conventional RFID system capable of reading EID tags is shown in FIG. 1. The reader includes circuitry, which generates an activation signal (usually a single frequency unmodulated signal) using a signal source 101 and an amplifier 102 to drive a resonant antenna circuit 103. This activation, signal is manifest as a time-varying electromagnetic field, which couples with the EID tag 105 by means of the electromagnetic field's magnetic field component 104. The EID tag 105 converts this magnetic field into an electrical voltage and current, and uses this electrical power to activate its internal electronic circuitry. Using any of several possible modulation schemes, the EID tag conveys binary encoded information stored within it back to the reader via the magnetic field 104, where the detector and utilization circuit 106 converts this binary code into alphanumeric format tag data 107 in accordance with some prescribed application.
The resonant antenna circuit 103 in FIG. 1 typically includes at least one inductive and one capacitive circuit element. Examples of resonant antenna circuits are shown in FIGS. 2 and 3. In each circuit, the capacitive element 201 is connected to the inductive element 202. Functionally, the inductive element 202 radiates a magnetic field when driven by a time varying signal. Conversely, when exposed to a time varying magnetic field, current flow is induced in the inductive element 202, resulting in a time varying signal that appears at the resonant antenna circuit's connection points. These phenomena are well known to be consistent with a principle of physics known as Faraday's Law.
FIG. 2 illustrates a series-wired arrangement of the inductor 202 and capacitor 201, and FIG. 3 illustrates a parallel-wired arrangement of these same components. Either arrangement may be used in an RFID reader depending on other design attributes, such as the configuration of Amplifier 102 and/or the Detector and Utilization Circuit 106. In either case, the selection of inductor and capacitor values are determined by the equation FR=½π √ LC, which is well known to those of ordinary skill in the art. For example, if the capacitance C of the capacitor 201 is 5 nF (i.e., 5×10−9 Farads) and the inductance L of the inductor 202 is 281 uH (i.e., 281×10−6 Henries), then the resonant frequency is expected to be 134.27 kilohertz (KHz). A variety of values for L and C can be combined to produce a particular resonant frequency FR, and the selection of specific values generally depends on typical design considerations.
Since the purpose of the inductor 202 is to radiate and sense a magnetic field, the inductor is typically designed to optimize this capability. For low frequency RFID systems, the inductor (which is regarded as an antenna) is most frequently realized as multiple turns of an electrically conductive material, such as copper wire. In some applications, the multiple turns of wire (which can have any imaginable geometric shape) can be wound on a dielectric core or bobbin, the core or bobbin having no influence on the resulting inductance value. Such an antenna coil is often referred to as an “air-core antenna”. Alternately, the multiple turns of wire can be wound on a ferrous core or bobbin, the ferrous material having a pronounced influence on the resulting inductance value of the coil, as well as an influence on the shape of the magnetic field that the inductor radiates and senses. Such antennas are commonly referred to as ferrite-core antennas.
In many electronic identification applications, the reader device and antenna comprise stationary elements, being fixed in location and capable of reading compatible EID tags when the tags enter the reading zone of the reader's antenna. An antenna associated with a stationary reader can be of any size and/or method of construction. A common example of an antenna used in conjunction with a stationary reader is a loop antenna.
A stationary reader including a loop antenna is shown in FIG. 4. The reader includes a stationary loop antenna 300 that is located proximate a conveyor 301 that carries objects 302 to which EID tags 303 are attached. As the objects pass by the reader's antenna and enter the reader's read zone, the reader's activation signal energizes the EID tag, whereupon the tag transmits its data contents to the reader. Another stationary reader is shown in FIG. 5, which is useful in livestock identification applications. The reader includes a fixed loop antenna 310 and each animal 311 is tagged with an EID tag 312, commonly in the form of an eartag. As the animals pass through a lane that restricts the animals to single file movement, the eartags are read sequentially by the stationary reader and loop antenna.
A stationary reader for use in livestock identification applications where animals stop and remain for some period of time is illustrated in FIG. 6. The reader 320 is connected to a number of stationary loop antennas 321 via a multiplexer 322. In the illustrated system, wires connect the wireless loop antennas to the multiplexer. When an animal 323 bearing an EID tag 324, such as an eartag, approaches the read zone of the loop antenna, identification information can be obtained from the EID tag by the reader. In a number of applications, an animal. will approach a loop antenna for a particular purpose, such as feeding, milking, branding, or immunizing. In these circumstances, the identification information associated with the animal's EID tag can be correlated with other data, such as milk metering, feed dispensation, physiological data assessment, etc.
In many of the stationary reader applications described above, reader equipment and installation costs are high and reader utilization rates are low. For example, the system shown in FIG. 5 is implemented using individual readers for each animal station, which is expensive and poses reliability consequences due to the multiplicity of readers. The number of readers can be reduced by using a single reader that is multiplexed with a group of antennas (see FIG. 6), however, this typically involves the cost of installing wiring between the loop antennas and the multiplexer and the cost of the multiplexer itself.
Portable readers are an alternative to stationary readers. A portable reader can be carried by a worker, who can then move between animals reading EID tags using the portable reader. A disadvantage that can be encountered when using a portable reader is that the worker is frequently operating at the posterior of an animal while the animal's EID tag is attached to the animal's ear (i.e. the animal's anterior). Therefore, use of a conventional portable reader to read an animal's EID tag can pose a physical accessibility problem and in some cases pose physical hazards to the worker.